This invention relates to an improved process for the preparation of crystalline cryolite which has a molar ratio Na:Al of 2.9:1 to 3.0:1 and has a particularly low adsorbed water content, i.e., a maximum water content of 8% by weight.
According to U.S. Pat. No. 3,897,543, in order to obtain well-crystallized cryolite having a relatively low water content and possessing the abovementioned molar composition, it is necessary to employ hydrofluoric acid, which is relatively expensive, and to carry out the process in an acid medium at a pH of between 0 and 1. Since the starting material is a solution of sodium chloride or sodium sulfate, the effluent has a high content of hydrochloric acid or sulfuric acid, and the process itself must be carried out in vessels made from corrosion-resistant alloys. The need to control the pH accurately by means of glass electrodes in the hydrofluoric acid medium is a further disadvantage.
The older Hungarian Pat. No. 157,309 also relates to the preparation of crystalline cryolite, the adsorbed water content of which is comparable with that of the abovementioned patent. After filtration of the end product, the water content is about 12 to 16% by weight, whilst after centrifuging it is 6 to 10% by weight. According to this prior art process, the molar ratio of the reactants NaF and AlF.sub.3 in the reaction mixture must be between 2.75 and 2.98, but preferably 2.90. As is stated expressly in one of the experimental examples, a pH value of 5 must not be exceeded under these conditions, because a higher pH results in poorer crystallinity of the end product.
According to British Pat. No. 643,570, again, acid pH values (below 3) should be maintained, though in this case, in order to prevent an unacceptably high degree of contamination of the end product with silica.
According to West German Auslegeschrift No. 1,088,035 synthetic cryolite, having a very low content of phosphorus compounds, is prepared, starting from aluminum fluoride, by keeping the pH value of the reaction mixture below 2 by means of mineral acids, the compounds which provide sodium and fluorine being employed at least in the stoichiometric amount required for the conversion of aluminum fluoride, but preferably in larger amounts, which are not defined in more detail. These measures serve to prevent the formation of sparingly soluble iron, titanium or aluminum phosphate complexes, by forming soluble fluorine complexes. The Specification does not refer to the possible water content of the end product. However, the latter is probably high, as emerges from the drying and calcination times, amounting to hours, required for the amounts of cryolite produced on a laboratory scale.
Surprisingly, it has now been found that a "stoichiometric" crystalline cryolite with an even lower water content than that of the material mentioned in the Hungarian Patent Specification may be obtained if a series of specific measures is observed, including, inter alia, that a specific, narrowly restricted molar ratio of Na:Al varying from about the stoichiometric ratio to a 20% excess of NaF, is maintained in the reaction mixture, the pH values, in contrast to the older process, being between 5 and 8. These conditions result in an end product having a water content of between 6 and 8% where suction filtration is used and 1.5 to 4% if centrifuging is used.